1. Field of the Invention
This invention concerns an apparatus and evaporator for metallizing foils in a vacuum roll coating plant with reactive metals such as aluminum, nickel, iron, cobalt, etc., and also concerns apparatus and evaporators for carrying out the process. Foil metallizing with reactive metals according to the invention is carried out by roll coating machines operated in vacuum. The invention represents a new development applicable to the art of vacuum roll coating.
2. Prior Art
The art of metallizing foils in vacuum is well developed. To mention but one of the publications relating to it, an article by H. Walter on pages 107 to 134 of the book edited by K. Stoeckhert and entitled "Veredeln von Kunststoff-Oberflachen" (Upgrading of Plastic Surfaces), published by Carl Hanser, Munich and Vienna 1974, describes the use of resistance heated ceramic or intermetallic evaporators or boats still currently used for evaporating onto moving foil. However, several advantages known to characterize other continuous metallizing processes are not achieved when using resistance heated evaporators. Thus, the mask shown in FIG. 5.11 of H. Walter's article applied therein to indirectly heated zinc evaporators, cannot be used on ceramic evaporators or boats. Nor are the latter available in sufficient length to cover the whole width of a modern foil. Several individually controlled ceramic boats become necessary, placed at a boat-to-foil distance of at least 100 mm and thus obviously cancelling the possibility of obtaining strictly identical condensate thickness over the whole width of the foil. Also, the evaporation can only take place in an upward direction, and even so, 70% of the metal evaporated is wasted, being dispersed in the vacuum apparatus where it becomes the source of harmful powder and stray deposits requiring frequent cleaning. Last but not least the evaporation temperature of resistance heated ceramic or intermetallic evaporators or boats cannot exceed 1450-1500.degree. C., which corresponds to a kinetic energy of the evaporant vapor of less than 0.15 electron volts (eV), and even this low and unsatisfactory kinetic energy is further reduced by the isentropic expansion of the vapor leaving the boat, and amounts to at most a few hundredths of an electron-volt when condensing on the foil. The result is a highly porous, gas and vapor permeable, humidity sensitive, often poorly-adhering condensate, the quality of which is very much inferior to that of deposits obtained from high kinetic energy vapor produced by sputtering or electron beam ion plating processes.
The invention aims at the elimination of the above disadvantages of ceramic or intermetallic resistance heated boats. By shaping the foil metallizing source as a cavity or hollow body from which the vapor of the reactive metal emerges as a beam, 80% or more of it can be condensed on the foil. By imparting a higher kinetic energy to the vapor (that no longer loses most of its kinetic energy by unwanted spreading), the quality of the condensate is considerably improved. The heater function is transferred from the electrically unstable multi-component ceramic or intermetallic boat to a radiation heater placed in the cavity and made of perfectly controllable high-melting-temperature metals or refractory conductors such as tungsten, tantalum, molybdenum, high-melting-temperature carbide or boride and the like. Any of these can be produced in considerable lengths and of uniform quality, whereby evaporators of any length can be built. By heating the radiation heater to a temperature sufficiently high to ensure a copious emission of electrons, some of these are attached to the vapor atoms of aluminum or to other vapor atoms or molecules known to have an affinity for electrons. Otherwise the emitted electrons and photons are useful for impact ionization of the vapor. The electrons emitted by the radiation heater can also be used for both impact ionizing the metallic vapor, and assisting the high energy thermal photons in vaporizing the liquid to be evaporated. In all these cases, the high temperature radiation heater provides the metallic vapor with high kinetic energy that can be further increased by the known process of accelerating the metallic vapor in an electric field.
As the temperature of the radiation heater is too high for a chemical reaction (corrosion) to occur with the vapor cloud surrounding it, the vapor cloud extends rather than shortens the useful life of the radiation heater, as can be seen in Langmuir's formula where it is shown that the increase of the pressure of a chemically inactive vapor diminishes the evaporation rate of the heater material. The usual electrochemical corrosion occurring in resistance heated ceramic or intermetallic boats is absent in radiation heated inner walls of cavities or hollow bodies that serve as a support for the liquid evaporant; their corrosion is consequently reduced, resulting in a particularly long-life evaporator.
SUMMARY OF THE INVENTION
It is an object of the invention to provide foil metallizing apparatus with indirectly heated evaporators instead of resistance heated ones when evaporating reactive metals such as aluminum. However, unlike the indirectly heated boats for evaporating zinc as shown in the aforementioned book at FIG 5.13, where the heater surrounds a boat or other liquid-containing receptacle, and wherein the radiated heat, i.e., the thermal photons, comes from the very wall of the cavity, the liquid according to the inventive process appears on and is borne by the inner wall of the cavity that is heated by the thermal photons originating from a radiation heater. The radiation heater also does not touch the reactive liquid evaporant but is placed in the cavity. The thermal photons heat the cavity material, namely a high-melting-temperature chemically-resistant, and preferably also good heat-conducting material such as boride or carbide or beryllium oxide or boron nitride or tungsten or the like. The cavity material in turn transmits its heat by thermal conduction to the liquid evaporant spread over it or over part of it or issuing from its pores or capillaries. Meanwhile the liquid evaporant thus brought to a higher temperature and mostly also increased photon absorption state, is also directly heated by said photons and is brought to evaporation.
In order to facilitate evaporant spreading over the inner surface of the cavity, the cavity is preferably provided with capillaries used for taking up and holding the liquid evaporant that is fed to the evaporator in foil or powder or conventional wire or liquid form. The shape of the radiation heater is preferably such as to provide for both melting and evaporation of the reactive metal. The inventive process thus is related to sublimation methods used to evaporate SiO, Cr, etc., and in particular is related to the subject matter of German Patent Application DE 35 30 106-A1 of Elektroschmezwerk Kempten GmbH, D 8000 Munchen, wherein continuous feeding of subliming materials is described. Instead of subliming solid material as in these methods, the present invention concerns the evaporation of molten reactive metals that could up to now not be continuously evaporated in foil coating plants by radiation heating methods because only when a molten reactive metal is borne by the inside of a sufficiently large cavity wall can the quantity of vapor required for metallizing foils at the usual speed of 6 to 12 meters/second be produced by a thermal radiation heating method.
An important object of the invention is to make the inside of the cavity or hollow body a particularly efficient photon-absorbing material. Most of the corrosion resistant refractory materials mentioned above are characterized by appreciable photon absorption at the temperatures concerned, but even the less photon-absorbent materials such as A1.sub.2 0.sub.3, BN, etc., can be made more photon receptive and absorbent by providing them with small cavities, i.e., pores and capillaries that are normally considered undesirable in resistance heated ceramic or intermetallic evaporators. Such highly photon absorbent material appears dark or gray, and it is therefore an object of the invention to make the inner layer of the cavity dark or gray, in addition to being corrosion resistant, of high melting temperature, and a good conductor of heat. This contrasts with the properties preferred for the outside of the cavity-defining structure that should for obvious reasons be as liquid impermeable, i.e., free from open pores and capillaries, as possible.
These objects are achieved in various ways. The inner layer can be made separate from an impermeable outer one, feeding the reactive metal between the two, so that the metal can permeate the inner porous layer from which it is subsequently evaporated. A dense outer layer can be separated from a porous inner layer by a middle layer preferably made of the same material as the inner layer, but having much larger pores, capillaries or channels to which the evaporant is fed by gravity, being melted and then passed through the finer pores of the cavity. A dense outer shell can be directly coated on its inner side with capillary and porous material. The inner side can be transformed by chemical or physical action into such material. All of a relatively thick walled cavity, for example 3 to 10 mm thick, can be made of the same porous material, and the reactive metal to be evaporated is then fed to the inner side on which is spreads and from which it evaporates, while the flow of the molten reactive metal to the outer side of the cavity or hollow body follows the rules of Juvin and Poiseuille, being considerably slowed down by increasing friction due to the decreasing temperature along the important temperature gradient over the thick porous cavity wall. Accordingly, only a fraction of the refractory metal fed to the inner side of the wall can evaporate from the outer side. The outer side of a porous shell can be made more dense or made impermeable by physical or chemical means. Other means are also possible.
A further object of the invention is to produce long evaporators covering the whole width of a foil to be metallized. This object can be achieved not only by producing single long specimens according to the invention, but also by lining up shorter specimens. The lined-up shorter specimens do not have to have perfect electrical contacts as required for resistance heated ceramic or intermetallic boats and therefore can be made into a line of any required length. The advantages of zinc evaporators, including the possibility of using exchangeable masks, are therefore realized, with the added advantage that the masks if necessary can also be used for carrying masts or hooks supporting long radiation heaters made of rods or coils or strips or the like. However, full length radiation heaters are not an absolute necessity when operating long evaporators, in view of the fact that radiation heaters made of tungsten and the like can be split up into strictly identical heating elements that in turn can be connected in series or in parallel or each connected to and heated by an individual power supply.
Another object of the invention is to concentrate most of the radiated heat in the cavity or hollow body, and thus to prevent thermal damage to the foil. In addition to the use of known methods such as heated vapor deflectors and the like, the new method of shaping the radiation heater as a cavity whose opening or openings are directed against the wall carrying the molten evaporant, has been found particularly efficient. According to this method, the wall carrying the molten reactive metal evaporant receives near-black body radiation, while the radiation from the exterior part of the radiation heater, i.e., the radiation originating from the part facing the outlet of the evaporant vapor, corresponds to the radiance of the heater material only, being less than one third of the black body radiation which would occur if tungsten heated to 2500.degree. C was used as radiation heater material. Another possibility is to set slits or small holes or other vapor dispensing openings of the cavity or hollow body in positions where they shade the foil, so that only the vapor, and not the direct radiation of the heater, reaches the foil.
The invention also makes it possible to co-evaporate several metals, and to put down several vacuum coatings on a foil supported by the same capstan roll. The former can be done by feeding to the same cavity several metals, or by feeding each metal to an individual cavity, while the latter is made possible by the facility with which openings in the cavity can be made in any direction, so that individual evaporators can be provided not only under the capstan roll, but also on both of its sides.